I honestly despair. So the overriding opinion is that mental illness is not an excuse for what he did and he's just evil. What experts we all are. I don't know and no-one else here knows whether he is or not, but if he is, it absolutely could be a reason for it, and doesn't make him evil.
While the vast majority of mentally unwell people (like me) are more at risk than pose a risk, mental illness can still be a reason why people do terrible things to others. It doesn't help our understanding and treatment of mentally unwell people to deny that
.
Equally, how, and if, you judge people's actions while mentally ill, must be different to how you'd judge the same actions by a well person. You don't judge someone's ability to walk with a broken leg the same as someone with two fit legs, so why do you judge the actions of a paranoid schizophrenic the same as a mentally well person?
That's not to say that society doesn't need to be protected from the risk a small minority of mentally ill pose but blame the condition, not the person.
Also, it's a well-worn trope in crime dramas, but just because someone has made plans, and took multiple steps to complete some atrocious crime, doesn't mean he can't be mentally ill.
Finally, we don't make society safer by putting punishment above all else and shouting "evil!". It doesn't help us prevent further deaths, it doesn't help us encourage people at the early stages of mental illness to seek help, it doesn't help society spot (accurately) the early signs of risk behaviour and source and supply preventative support, it just fulfils our anger and need for vengeance.
Yes, he might be a terrible person, and deserve every punishment he receives, or he might be significantly mentally unwell, with a perfect storm of life events and experiences that have coalesced to create trigger the terrible actions he committed. None of us know, but calling him evil and thinking no more about it, and putting a need for vengeance above a need to understand, won't save future children.